One of the objects of the invention is to provide a display device of the type mentioned in the opening paragraph, in which the angle-dependence is reduced considerably. A further object of the invention is to provide a compensator layer which can be used, inter alia, in such display devices.
Therefore, a display device in accordance with the invention is characterized in that the display cell comprises at least two retardation foils which predominantly contain polymerized or vitrified liquid-crystalline material, the liquid-crystal molecules in the polymerized liquid-crystalline material exhibiting a tilt angle relative to the substrates, and the average directions of orientation of the liquid-crystal molecules in the polymerized or vitrified liquid-crystalline material of each of the retardation foils making an angle with each other which ranges between 60 and 120 degrees, viewed at right angles to the substrates.
The polymerized, liquid-crystalline material may be partly polymerized, but, preferably, it is polymerized substantially completely.
In this context, the direction of orientation of a liquid-crystal molecule is to be understood to mean the perpendicular projection on a substrate of the director of the liquid-crystal molecule. A retardation foil is to be understood to mean a birefringent foil or layer or a foil or layer having an optically compensating or delaying effect (an optically anisotropic layer).
The invention is inter alia based on the recognition that as a result of the tilt angle of the liquid-crystal molecules in the polymerized or vitrified liquid-crystal material, the retardation of one of the retardation foils compensates, as it were, for the retardation of a part of the liquid-crystal molecules in the display cell in the driven state; the retardation of the other retardation foil compensates, as it were, for the retardation of another part of the liquid-crystal molecules in the display cell in the driven state.
The invention is further based on the recognition that such layers of a polymerized liquid-crystal material can be manufactured in a simple manner, for example, by xe2x80x9cspincoatingxe2x80x9d of nematic liquid-crystal materials or by polymerization in the smectic C phase or by vitrification. Dependent upon the manner in which they have been manufactured, the liquid-crystal molecules in the polymerized liquid-crystalline material exhibit a tilt angle relative to the substrates, which varies (for example by using surface-active materials) or which is practically constant. This can be determined by means of conoscopy or microscopy using polarized light (polarizing microscopy).
A preferred embodiment of a display device in accordance with the invention is characterized in that the direction of orientation of the liquid-crystal molecules in the polymerized or vitrified liquid-crystalline material is substantially constant in at least one of the retardation foils.
In a liquid-crystal display device in which the customary voltage is applied across the liquid-crystal material in the on-state, the directors in this material actually still make a small angle with the direction perpendicular to the substrates. As a result, the birefringence is different at different viewing angles and non-symmetrical with respect to a direction perpendicular to the two substrates, which explains the angle-dependence of a liquid crystal having a nematic structure. This birefringence is caused, as it were, by two partial layers of a liquid-crystal material in which the optical properties of a partial layer are governed by the average tilt angle in the relevant partial layer with respect to the substrates and by the average direction of orientation; if a sufficiently high voltage is applied across the liquid-crystal layer, then the average direction of orientation in the partial layer is approximately equal to the direction of orientation, as determined by the orientation layers on the substrate, so that the magnitude of the difference between the directions of orientation of the two partial layers is practically equal to the tilt angle. In accordance with the invention, the angle-dependence can be substantially eliminated by causing the average directions of orientation of the liquid crystal molecules in the polymerized liquid-crystalline material of the retardation foils to intersect each other at an angle which is practically constant (for example equal to the twist angle of the display cell). The tilt angles of the liquid-crystal molecules in the polymerized liquid-crystalline material can be set relative to the substrates, such that the compensator composed of the retardation foils compensates the angle-dependence of the cell substantially completely.
During the manufacture of a retardation foil, the tilt of the liquid-crystal molecules (director profile) can be obtained by using a polymeric (or vitrified) material which is formed from a liquid-crystal monomer.
In principle, any liquid-crystalline polymeric materials can be used to produce the material for the retardation foils. However, use is preferably made of liquid-crystalline polymeric materials which are the reaction product of monomers or of a mixture of monomers comprising a reactive group. Such polymeric materials have the advantage that the liquid-crystalline groups can be oriented prior to polymerization. Polymerization causes such an orientation to be frozen as it were. It is noted that such a mixture may also comprise non-reactive liquid-crystalline monomers. The reactive monomers preferably comprise a liquid-crystalline group.
For the reactive group use can be made of vinyl ethers, thiolene systems or epoxy groups. However, use is preferably made of reactive groups in the form of (meth)acrylate groups. Monomers comprising a(n) (meth)acrylate group proved to be excellently processable. In principle, the monomers can be thermally polymerized. In practice, radical-polymerization under the influence of actinic radiation, in particular UV light, is the simplest way of polymerizing the monomers. This has the advantage that persons skilled in the art can choose the temperature at which the mixture should be polymerized themselves. The choice of the temperature is often very important as the liquid-crystalline properties of the mixture to be polymerized are governed to a substantial degree by the temperature.
Preferably, the mixture to be polymerized also comprises monomers having two or more reactive groups of the above-mentioned type. During polymerization, the presence of such monomers leads to the formation of a three-dimensional network. This causes the optical properties of the inventive retardation foil to become less sensitive to variations in temperature. In particular for foils which are employed at different temperatures, such a small temperature-dependence of the optical properties is very favorable.
Liquid-crystalline molecules which can be used within the scope of the invention correspond to the general formula
A-B-M-(B)-(A)
In this formula, M represents a liquid-crystalline group. Suitable M groups are disclosed, inter alia, in U.S. Pat. 4,398,803 and WO 95/24454. B represents a so-called spacer group. Dependent upon the desired properties, the monomers used comprise one or two spacer groups. Spacer groups are also known from the above-mentioned Patent publications. A represents a reactive group of the above-mentioned type. The liquid-crystalline molecules may comprise one or two reactive groups. As stated above, a part of the liquid-crystalline molecules in the mixture may be non-reactive. In that case, these molecules do not comprise A-type groups.
A preferred embodiment of the display device is characterized in that the polymerized material comprises liquid-crystalline molecules which are provided, at one end, with a non-polar group and, at the other end, with a polar group. The presence of this type of liquid-crystalline molecules causes the liquid-crystalline material of the mixture to be polymerized to assume the homeotropic phase at a short distance from the substrate. As a result, the desired ordering of the tilt in the liquid-crystalline material of the retardation foil takes place almost spontaneously. Consequently, in this case treatments with electric fields to induce said tilt are redundant. This simplifies the manufacture of such foils.
Liquid-crystalline molecules having a polar end and a non-polar end correspond to the general formula
R-B-M-Z
where B and M have the above-mentioned meaning. In this case, the spacer group B serves as the non-polar group of the molecule and Z represents a polar group, such as xe2x80x94CN, xe2x80x94OH, xe2x80x94NO2, xe2x80x94COOH or xe2x80x94C(O)OCH3. R represents a further substituent.
A further preferred embodiment of the display device is characterized in that at the end provided with the non-polar group, the liquid-crystalline molecules are covalently bonded to the polymerized or vitrified material. This is achieved if for R use is made of a reactive group of the above-mentioned type. By virtue of this measure, the optical properties of the inventive retardation foil become less sensitive to variations in temperature. In particular for foils which are employed at different temperatures, such a small temperature-dependence of the optical properties is very favorable.
The tilt may be substantially uniform. Alternatively, during the manufacture of the display device, a pretilt can be induced in one or both boundary surfaces, for example by means of the method described in U.S. Pat. No, 5,155,610. Dependent upon this pretilt, the optically anisotropic layer may exhibit, for example, a xe2x80x9csplay deformationxe2x80x9d. The eventual director profile can also be influenced, during the manufacture, by means of electric and/or magnetic fields. This may result, for example, in a preferred direction for the directors. Such a preferred direction can alternatively be attained during polymerization in the smectic C-phase of liquid-crystalline materials.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.